There are conventionally known route guiding systems that guide people along a route by providing them with map information and other information. For example, navigation methods that rely on mobile communication involving a mobile device, incorporating a global positioning system receiver, and a control station, incorporating a database of map information and other information, work in the following manner. The mobile device transmits to the control station location information and destination information, based on which the control station then retrieves from the database a map around the current location and a route to the destination location. The thus retrieved route information is transmitted back to the mobile device and is displayed on the display section thereof. This permits route guiding to be performed with enhanced accuracy, permits the user to acquire the information they need whenever they need it, and permits the user to be provided with various kinds of information that they may need other than map information (see, for example, Japanese Patent Application Laid-Open No. 2000-213952).
Some conventional navigation systems relying on communication involve a route guiding device, built as a car-mounted or hand-held unit and functioning as a terminal station, and a route information providing device, built as an information center and functioning as a base station, and these navigation systems work as follows. Between the route guiding device and the route information providing device, data such as location information and route guiding information is exchanged through communication so that the route guiding device is fed with information about a route to the destination location. In this way, route guiding is achieved. Here, of the retrieved route, not all but only part that the route guiding device has not yet traveled is selectively exchanged (see, for example, Japanese Patent Application Laid-Open No. 2000-18955).
However, the method disclosed in Japanese Patent Application Laid-Open No. 2000-213952 mentioned above has the following disadvantages. It is all the map information related to the retrieved route that the control station transmits to the mobile device. Thus, a large quantity of data needs to be transmitted through a lengthy communication session, resulting in increased communication costs and long processing time. In particular, in cases where the distance from the current location to the destination location is long, the increases in communication time and communication costs are too large to ignore. Moreover, the lengthy communication session tends to invite heavy communication traffic, leading to trouble such as garbled or aborted communication.
On the other hand, communication-dependent navigation systems like the one disclosed in Japanese Patent Application Laid-Open No. 2000-18955 mentioned above have the following disadvantages. The user often remembers, as known routes, routes that they have ever traveled, and therefore they are unlikely to have much difficulty to reach a destination if provided with only the map information related to the part of the route thereto that they have never traveled before, even if they are not provided with the map information related to the known part thereof. With this system, however, even when the user seeks route guiding but do not actually head for the destination, the retrieved route will be, when the same route is retrieved next time, treated as a known route. Thus, the user cannot acquire the related map information. This can be prevented by notifying the route information providing device of the routes that have actually been traveled. This, however, requires that the information related to the ever traveled routes be transmitted to the route information providing device, resulting in extra communication costs.
Incidentally, some car-mounted navigation apparatuses have guide information related to sightseeing spots, gas stations, and the like stored on a recording medium such as a DVD. For the propose of complementing such guide information in terms of quantity and updatedness, it has been proposed to download additional and updated guide information through communication (see, for example, Japanese Patent Application Laid-Open No. H11-266329). It has also been proposed to add advertisements to such guide information. It has further been proposed to show such guide information on a small screen such as those on cellular phones and PDAs (personal digital assistants). These proposals, however, greatly increase the quantity of guide information handled, now containing advertisements, and thus make it uncomfortable for the user to view the desired guide information, particularly when it is shown on a small display screen.